1. Field of the Invention
This invention relates generally to a protective coating for electrical devices and in particular to a thin film useful as a passivating coating for a semiconductor device.
2. Description of the Prior Art
In present day semiconductor technology, passivation layers are used to protect semiconductor device structures against environmental influences that arise in the manufacture and use of semiconductor devices. Passivation layers protect the semiconductor devices from the effects of moisture and contaminants during the manufacturing process and thereafter, or during the operation of the devices in circuit environments. By using passivation layers, production yield is increased and deleterious effects are minimized when the semiconductor device is operating in the field.
Especially desirable are passivation layers that effectively passivate surface states, which are electron energy levels at the surface of the semiconductor substrate, characterized by electrical charge and discharge having variable time constants. This phenomenon causes short term and long term electrical drift, which undesirably changes the characteristics of a field effect transistor, and other phenomena such as light sensitivity and noise generation. Surface states are "passivated" when a layer of material (typically but not necessarily an insulating material) overlying the semiconductor surface interacts with atoms at the surface in such a way as to reduce the time constants characterizing the electrical charge and discharge of the surface states to values small enough to eliminate electrical drift problems. Surface states on a silicon crystal can be passivated by a silicon dioxide layer produced by thermal oxidation, for example. Surface states on gallium arsenide can be passivated by a layer of semiconducting material other than gallium arsenide, for example, provided the interface between the gallium arsenide surface and the semiconducting material is appropriately controlled. The resulting junction between the semiconductor surface and the layer of semiconducting material is called a "heterojunction".
Passivation layers often insulate and protect the semiconductor surface against electrical shorting and low breakdown voltages. Passivation layers also act as potting materials that protect against surface scratches and thus prevent electrical shorting.
Some semiconductor device handling processes use vacuum wands or other tools to move wafers or chips from one position to another. In such cases, the tool may move or abrade exposed metal conductors that are formed on the wafers. Passivation layers help eliminate this problem. Another problem is encountered with semiconductor devices that incorporate an air bridge, which is a metal connection to a metal conductor that skips over an adjacent conductor, so that significant capacitive coupling is not added between the two conductors. The air between the air bridge and the skipped-over conductor has a low dielectric constant of nearly unity. However, if the metal air bridge is subjected to mechanical pressure causing it to contact the skipped-over metal conductor, an electrical short would result. A passivation layer overlying the skipped-over metal conductor can act as an electrical insulator to insure against such shorting.
Passivation layers generally are composed of silicon dioxide or silicon nitride, for example. Passivation layers using such materials are usually relatively thick, about 2000 .ANG. or more, and require long deposition time, which adds to the cost of the semiconductor devices. With dielectric constants more than three times that of air these thick layers increase the capacitances between various parts of a semiconductor device, thereby degrading the device's high-frequency performance. Also, silicon dioxide and silicon nitride do not adhere very well to gold which is used for electrodes or conductors and actually are known to separate from gold conductors so that circuit problems are caused. Thick layers of insulators or passivation material using silicon dioxide or silicon nitride are subject to strain, and as they are relatively brittle in nature, can experience cracking and do not seal well. Furthermore, insulators such as silicon dioxide or silicon nitride do little to passivate surface states on some semiconductors, such as gallium arsenide, and can themselves act as charge traps and produce drift problems.
In the Japanese Patent Application No. 58-145134 filed Feb. 23, 1982 by Toshiaki Ogata, diamond films formed by thermal decomposition of hydrocarbon gases in a depressurized atmosphere or by plasma formation are used as the insulating films between polycrystalline silicon wiring and aluminum wiring. Diamond films formed by the same method are also used in passivation layers above the aluminum wiring. The diamond films in the passivation layers are reported to improve heat dissipation and also improve mechanical strength. The term "passivation" here is not used in the context of a layer overlying the semiconductor surface which interacts with atoms of the surface in such a way as to effectively passivate the surface states of the semiconductor substrate.
Ogata does not describe the thermal decomposition process or the plasma formation process, however, a typical thermal chemical vapor deposition (CVD) apparatus used to deposit continuous films of diamond at rates up to 10 micrometers per hour, is shown in FIG. 1 of this application. In this CVD procedure, a deposition chamber 110 includes a tungsten filament 100 that is heated to 2000.degree. C. to dissociate gaseous hydrogen supplied through pipe 101 that is disposed above a substrate 103, which may be of a silicon, silica, or molybdenum material. The substrate 103 rests on a molybdenum cover 104 which in turn is supported by a substrate holder 105. A mixture of methane and gaseous hydrogen is supplied through pipe 101 so that both the methane and the hydrogen flow over the tungsten filament 100 before reaching substrate 103. The deposition chamber 110 is surrounded by a silica tube 106 that is attached to furnace 107.
In the thermal CVD process using hydrogen and methane, the carbon atoms from the pyrolyzed methane are deposited upon the heated substrate 103 where they build up as polycrystalline diamond deposits. In another prior art thermal CVD process, the gaseous hydrocarbon, such as methane, is replaced by methanol, ethanol, acetone or some other oxygen-and/or nitrogen-containing carbon compounds. It has been observed that the diamond deposition rate for this prior art thermal CVD process was at least ten times that of the methane based techniques, indicating that these other compounds are more efficient than the methane. With acetone, the polycrystalline films grew at a rate of 10 micrometers per hour.
A typical plasma formation apparatus for creating a diamond-like film on transparent slides of silicon or silica is shown in FIG. 2. A mixture of hydrogen and methane enters through the gas inlet 200 into a fused quartz tube 205. The quartz tube 205, which serves as the deposition chamber, is evacuated by a pump 206. As the hydrogen-methane mixture progresses through the tube 205, the mixture is passed through a microwave discharge 201 that dissociates the gas molecules to form an electrically charged plasma 204. The plasma 204 comes into contact with a sample 202 to be coated, which is supported by a stand 203 within tube 205. Single atoms of carbon bombard the surface of sample 202 and gradually the carbon atoms on the surface link together into diamond crystal lattice structures.
In each of the above methods the important aspect of the process is the dissociation of the hydrogen molecules into unstable atomic hydrogen. The presence of atomic hydrogen is believed to prevent double bonds from forming between carbon atoms until the carbon atoms are ready to join together in the single bonded configurations required for diamond crystals.
In both of the above prior art methods for forming a diamond-like film, the hydrogen and carbon are pumped into the deposition chamber as gases and are dissociated by the process within the chamber. The silicon substrate is subjected to high temperatures and high energy particle bombardment. If a compound semiconductor, such as GaAs, is used, the high energy hydrogen atoms penetrate the compound semiconductor substrate which causes a reduction of carrier concentration near the surface due to compensation. Accordingly, the prior art methods of depositing a thin diamond layer on a substrate are not suitable for use with compound semiconductor substrates such as GaAs. A novel and improved method, which is suitable for deposition of a thin diamond layer on semiconductor surfaces of a substrate wherein the substrate is not exposed to high energy particles, is disclosed below.